1. Field of the Invention
The present invention relates to polyurethane resins which are useful in producing polyurethane resin coatings which, in particular, exhibit excellent weather resistance.
2. Description of the Prior Art
Polyurethane resins are classified into two types by the kind of isocyanate compounds contained in them. The classification thus includes "yellowing" and "non-yellowing" types. Isocyanates which have hitherto been in use as ones giving polyurethane resins of the non-yellowing type include: such aliphatic isocyanates as hexamethylenediisocyanate, isophoronediisocyanate, 2,2,4-trimethylhexamethylenediisocyanate, dicyclohexylmethanediisocyanate, etc.; and xylylenediisocyante, etc. Whilst non-yellowing type polyurethane resins obtained by causing these isocyanates to react with polyols have been employed as coating materials, there still remained many problems to be solved in using them for such purpose.
One of these problems is their toxicity. Since isocyanate compounds are substances which are chemically highly reactive, they are very dangerous when used by persons who are of an allergic constitution or have weak respiratory organs. Because of this, the limit of concentration in the atmosphere of, for instance, monomers of toluenediisocyanate, diphenylmethanediisocyanate, etc., was fixed at 0.02 ppm by the Commission of the American Conference of Governmental Industrial Hygienists. For such reason, the unmodified isocyanate monomers are seldom used in polyurethane coatings. However, in special cases, they are used after having been modified into prepolymers--adducts obtained by adding them to trimethylolpropane, ethylene glycol, etc. This modification has the effect of lowering vapor pressure, thereby reducing the toxicity and bad odor of the isocyanate. In addition, modification allows adjustment of the reactivity of the isocyanate and diversification in the type of coatings to be realized.
Since, however, it is difficult to wholly eliminate isocyanate monomers in the prepolymer additions during commercial operation, the fact is, that one still smells a strong irritating odor while he is engaged in the work of preparing coating materials or of applying coats. Many people complain of the symptoms of respiratory diseases which are peculiarly contracted by inhaling isocyanate vapors. With an increase in the use of polyurethane products, this question has now more than ever been brought much to the fore.
With the isocyanate additions and while these materials are in storage, in particular, it is said that there occurs dissociation of diisocyanates into highly toxic monomers, depending upon the storage conditions, and this constitutes an uneasy factor for those concerned including chemical engineers and operators alike. Under such circumstances, certain measures are being taken for improvement of the working environment. For instance, provision of good ventilation, is necessary so that the operators will not directly inhale the isocyanate compound vapors. However, the state of things in this connection is still far from being satisfactory.
The second question in the conventional technology is that there remains, with the conventional coatings, much to be desired with respect to their weather resistance. Although the non-yellowing type polyurethane resins were originally developed with a view to improving the weather resistance of coatings, they are not, as yet, sufficiently resistant to weather when used as coatings for items which are exposed to severe outdoor conditions over a long period of time, such as automobiles, railroad carriages, aircrafts, vessels, building materials, and so forth.
The third question concerning polyurethane coating materials, according to conventional techniques, is that the range in which the selection of solvent composition can be made is not necessarily wide enough, and that, if the amount of solvent is reduced from the viewpoint of energy saving and prevention of environmental pollution, the coating efficiency of the resultant product, being the so-called "high solid type" coating with a high concentration of polyurethane resin, is impaired because of its high viscosity.